Mayer has been serving as vice president of Local, Maps & Location Services. However, even though she has been with the Mountain View, Calif.-based enterprise since 1999 when she joined as Google's 20th employee, The New York Times reports that she wasn't on track to be elevated to a C-level executive.

Taking that into account, it wouldn't be a surprise that Mayer was looking to move up elsewhere.

While it is a major milestone for her career (not to mention she's joining a growing list of female CEOs at Silicon Valley powerhouses), Mayer is defintiely going to hit the ground running at Yahoo. She starts Tuesday.

When Scott Thompson stepped into the CEO role at Yahoo earlier this year, ZDNet's Larry Dignan outlined five big challenges that Thompson would have to address.

Relevance: Yahoo has fallen behind in search, but it keeps trying to play up the digital media company angle. One of these needs to grow in order to make Yahoo relevant to the general public again.

Innovation: As an executive coming in from Google, the pressure is going to be on for Mayer to apply whatever she's learned with the Internet giant and apply it in a whole new way for Yahoo's benefit. Given Google's reputation for thinking outside the box and resolving big problems in simple ways (most of the time), let's hope she can bring some of that magic to Yahoo.

Definition: This ties in with relevance, but Yahoo needs to rebrand itself. Originally a search giant itself, even a partnership with Microsoft's Bing hasn't really dented Google's market share. Based on comScore figures from June, it looks like only Bing is getting anything out of this dea. Mayer is going to need to rework how the Bing deal could benefit Yahoo -- or just move on to the digital media angle entirely.

Content: Yahoo has been stepping it up with original digital media content, but perhaps the Cupertino, Calif.-based company should ramp up its entertainment partnerships to really get this moving along and feed content in more quickly.

Morale: The only major tech companies where morale might be lower would be RIM and AOL. Yahoo, however, still has a lot more potential to turn itself around. However, Yahoo has gone through a handful of CEOs in the last few years, and the last three (Jerry Yang, Carol Bartz, and Thompson) have all left on uneasy terms, to say the least. Is the Yahoo CEO job cursed? Unlikely...unless you believe in that sort of thing.

Mayer commented lightly in prepared remarks about her future gig, "Yahoo!’s products will continue to enhance our partnerships with advertisers, technology and media companies, while inspiring and delighting our users. There is a lot to do and I can’t wait to get started."